The long-term objectives of this project are to reveal how the visual pigment rhodopsin converts light signals into chemical messages that electrically polarize photoreceptor cells. Crucial evidence will be obtained by determining the three dimensional atomic structure of dark adapted rhodopsin and, ultimately, of light adapted forms of rhodopsin. Ideally, the three dimensional structure of the complex between light excited rhodopsin and the retinal rod GTP binding protein will also obtained. The project may also reveal the three dimensional atomic structure of bacteriorhodopsin and provide information on its mechanism of transmembrane pumping. The specific aims are to find reliable conditions to grow three dimensional crystals of rhodopsin and/or bacteriorhodopsin that are suitable for x-ray diffraction to atomic resolution. Diffraction patterns will be measured to evaluate the quality of the crystals. Diffraction will also be measured on light-adapted rhodopsin crystals that preliminary experiments suggest resemble metarhodopsin I. Bacteriorhodopsin appears to have structural features similar to rhodopsin in the oily hydrocarbon region of the membrane (cf. Dratz and Hargrave, TIBS, 8, 128-131, 1983). It is possible that bacteriorhodopsin will be an easier case to crystallize than rhodopsin because it is more compact and is extremely stable in the dark and light. Structural information on bacteriorhodopsin is important in its own right and presumably will be strongly suggestive for rhodopsin structure. Rhodopsin and bacteriorhodopsin will be purified free from their native lipids in suitable detergents and concentrated. Controlled amounts of specific detergents or lipids and/or other amphiphiles will be added and conditions manipulated to slowly lower the solubility of the proteins to form crystals either by dialysis or by vapor diffusion. Determination of the atomic structure of rhodopsin (and/or bacteriorhodopsin) is extremely important for a fundamental understanding of visual excitation but must certainly be considered a high risk, pilot project at this time.